This invention concerns solidified radiocative wastes and a process for producing the same and, more specifically, it relates to solidified radioactive wastes suitable to stable immobilizing of radioactive wastes containing water soluble solid components for a long time, as well as a process for producing the same.
For the stable storage or disposal of radioactive wastes resulted from those facilities for handling radioactive materials such as nuclear power plants, it is necessary to pack and immobilize the radioactive wastes together with a solidifying material within a container so that radioactive materials may not diffuse to the surrounding environment.
Among the radioactive wastes, those resulted from BWR type reactors mainly comprise sodium sulfate and ion exchange resins and those resulted from PWR type reactors mainly comprise sodium borate. Among them, sodium sulfate and sodium borate are highly water soluble and the ion exchange resins are swollen upon absorption of water. Accordingly, the mixing ratio of radioactive wastes can not be increased and no stable solidification products can be formed in the case of using water setting inorganic solidifying materials such as cements since water has to be used in the admixture therewith. Further, since residual water in the thus formed solidification products leaves passages (open pores) upon evaporization to increase the porosity therein, the leaching rate of the radioactive materials is increased.
It has been developed a pellet solidifying process in which pelletized radioactive wastes such as of sodium sulfate are mixed with solidifying material to improve the mixing ratio of the radioactive wastes. However, also in this process, bulky pellets may absorb water to swell in the case of using a water setting solidifying agent that requires water. In addition, the water setting solidifying agent may possibly result in cracks due to the uneven shrinkage caused by the curing reaction that lasts for a long period of time and if the cracks are developed as far as the water soluble pellets, intended immobilizing effect for the radioactive material by the solidifying material can no more be attained.
While on the other hand, a technique of utilizing organic asphalts or plastics as the solidifying material has been developed and partially put to practical use. However, since these organic solidifying materials require heating upon their solidification, they may cause decomposition or degradation and they are not always excellent in the heat resistance and radiation stability.
In view of the above, the present inventors have made a study on inorganic solidifying material having excellent compatibility with water soluble material and, as the result, have found that a solidifying material comprising the solution of an alkali silicate can prevent the leaching of water soluble radioactive wastes and immobilize them effectively. Although the solidifying agent composed of the alkali silicate solution comprises an inorganic material and is excellent both in the heat resistance and radiation stability, it is insufficient in the water proofness and causes uneven shrinkage tending to induce cracking in the same manner as cement material.